Self-cleaning (that is pyrolytically cleaned) ovens incorporate several features, including:
Initial application of high heat at the top of the oven chamber to initiate operation of the catalytic smoke eliminator before heavy soils on the side and bottom walls are volatized. PA1 Continued supply of heat to maintain at least a minimum required temperature in the oven for pyrolysis of the soils over a period of time. PA1 Control of the heat to prevent the temperature from exceeding the operating or softening temperature of the enamel on the oven walls while maintaining the temperature of all parts of the oven walls within the effective self-clean range.
Manufacturers have used a number of different techniques to control the self-cleaning cycle. Typically, however, oven controls begin the cycle with full power (i.e. 240 v, 208 v, 120 v etc.) applied to the broil (upper) heating unit for a fixed amount of time. At some point during the cycle some controls switch to the bake unit as the primary source of heat input, while others use the bake unit to augment the broil unit input. This may be done at full power or at reduced power (i.e. 120 v etc.). Some other manufacturers use a fixed setting cycle switch, such as a bimetal switch for example, to reduce the effective power of the heating units. Other controls use one unit, either the bake or the broil, exclusively for the heat input. In all ovens known to the applicants, however, a thermostat is used to call for heat when needed to satisfy the minimum requirements and to stop heat input to keep the oven liner temperature from exceeding maximum design temperature.
In order to satisfy the requirements of the various normal cooking modes, the broil and bake units normally are high wattage elements. As a result localized temperatures adjacent the elements are significantly higher than the general level of the temperature in the oven. In a self-cleaning cycle this can cause the temperature of the enamel adjacent the heating element to exceed the operating range of the porcelain enamel. If the localized temperature approaches the glass transition temperature of the enamel/steel composite, the enamel may craze, crack or peel.
On the other hand, some portions of a typical oven liner generally have lower temperatures than the temperature prevalent in most of the chamber. For example, in ovens with a drop down access door, the area adjacent the lower edge of the door often has lower temperatures. When prior art controls keep the maximum temperatures within the desired range, such lower temperature liner areas often drop below the self-clean range and their cleaning is not fully satisfactory.
In order to balance the need to provide as short self-cleaning cycles as practical and the need to stay below the degradation temperature of the oven walls, the self-cleaning cycle should be conducted within a rather narrow temperature band. For example an optimal cleaning operation would result from maintaining all portions of the cooking chamber walls at a constant 880.degree. F. for about three hours. Reasonable balanced results can be obtained by operating between 800.degree. F. and 950.degree. F. However, in order to maintain the overall oven wall temperatures in this range, prior art controls often resulted in localized enamel temperatures outside the range and high enough to adversely affect the enamel, particularly near the broil element and the lower wall under the bake element.
It is an object of this invention to provide an improved self-cleaning oven in which the enamel covering of the liner is not adversely affected by the cleaning operation.
It is another object of this invention to provide such an improved oven in which localized temperatures of portions of the liner do not become deleterious to the enamel and yet effectively clean the entire oven liner.
It is yet another object of this invention to provide such an improved oven in which the designated heating units are energized in a cyclical fashion to moderate the localized temperatures.
It is still another object of this invention to provide such an improved self-cleaning oven without the need for additional hardware devices.